Frequency spectrum analyzers are widely used for analyzing frequency spectrum of an incoming signal in a frequency domain. Typically in such a frequency spectrum analyzer, levels of frequency spectrum are displayed in a vertical direction with respect to a frequency range in a horizontal direction. A frequency spectrum analyzer includes three or more frequency converters connected in series each of which is formed of a frequency mixer, a local oscillator and a band pass filter to produce intermediate frequency (IF) signals without image (spurious) responses.
An example of conventional frequency spectrum analyzer is shown in FIG. 3. The frequency spectrum analyzer of FIG. 3 includes three frequency converters. The first frequency converter is formed of a first frequency mixer 11, a first IF filter 21 and a first local oscillator 31. The second frequency converter is formed of a second frequency mixer 12, a second IF filter 22 and a second local oscillator 32. The third frequency converter is formed of a third frequency mixer 13, a third IF filter 23 and a third local oscillator 33. The frequency spectrum analyzer further includes a ramp wave generator 50, a detector 60 and a display 70.
Typically, the first local oscillator 31 is a sweep frequency oscillator whose frequency is linearly swept by a ramp wave from the ramp wave generator 50. The second and third local oscillators are fixed frequency oscillators. The frequency of the first local signal is higher than that of the second and third local signals.
An input signal F1 to be analyzed is mixed with the first local signal by the first frequency mixer 11, thereby producing first IF signals having both sum and difference frequencies between the input and first local signals. The first IF filter 21, which is a band pass filter, selects either one of the sum or difference IF signals from the first frequency mixer 11.
Thus, the first IF signal is provided to the second frequency mixer 12 where it is mixed with the second local signal from the second local oscillator 32. The second frequency mixer produces second IF signals having both sum and difference frequencies between the first IF signal and the second local signal. The second IF filter 22, which is a band pass filter, selects either one of the sum or difference IF signals from the second frequency mixer 12.
Similarly, the second IF signal is provided to the third frequency mixer 13 where it is mixed with the third local signal from the third local oscillator 33. The third frequency mixer produces third IF signals having both sum and difference frequencies between the second IF signal and the third local signal. The third IF filter 23, which is a band pass filter, selects either one of the sum or difference IF signals from the third frequency mixer 13.
The third IF signal from the third IF filter 23 is provided to the detector 60 where a DC voltage proportional to the AC power level of the third IF signal is produced. The DC voltage is provided to the display 70 where it is displayed in a vertical axis as a power level. Since the ramp wave is also applied to the display 70 for driving a horizontal axis thereof, the display screen shows frequency spectrum of the input signal F1 in a frequency domain. In such a frequency domain analysis, the power level is shown in the vertical direction while the frequency range (span) is shown in the horizontal direction.
As briefly mentioned above, a frequency spectrum analyzer employs such multiple stages of frequency converters for eliminating image frequencies (spurious responses) by selecting appropriate frequencies in the local signals and IF signals. Further to eliminating the spurious responses, it is also important for a frequency spectrum analyzer to have a high carrier wave to noise (C/N) ratio to analyze an input signal with high sensitivity and resolution.
As is well known in the art, a C/N ratio of a spectrum analyzer is determined by C/N ratios (purity) of local signals used therein. This is because phase noise of local oscillators is usually larger than noise floors of other components in the spectrum analyzer. It is also known in the art that a C/N ratio of a fixed frequency oscillator is higher than that of a sweep frequency oscillator. Further, an oscillator having a highly selective resonant circuit such as a crystal oscillator has a higher C/N ratio than other types of oscillators.
In the arrangement of FIG. 3, the first local oscillator 31 is a wide range sweep oscillator typically using a YIG (yttrium-iron-garnet) resonator. The second and third local oscillators 32 and 33 are fixed frequency oscillators. A crystal oscillator with high stability is usually used as the third local oscillator 33. Thus, generally, degrees of phase noise in the first to third local oscillators will be expressed in the following order: EQU .phi..sub.LO1) .phi..sub.LO2) .phi..sub.LO3 (1)
where .phi..sub.LO1 denotes the phase noise of the first local oscillator 31, .phi..sub.LO2 denotes the phase noise of the second local oscillator 32, and .phi..sub.LO3 denotes the phase noise of the third local oscillator 33.
When the noise floor of other components in the spectrum analyzer is lower than the phase noise of local oscillators, and the phase noise of the local oscillators is random noise, overall phase noise .phi..sub.N of the frequency spectrum analyzer will be expressed as follows: EQU .phi..sub.N =((.phi..sub.LO1).sup.2 +(.phi..sub.LO2).sup.2 +(.phi..sub.LO3).sup.2).sup.1/2 (2)
Since the phase noise of the first local oscillator 31 is the largest, the equation (2) is written to: EQU .phi..sub.N .apprxeq..phi..sub.LO1 (3)
Thus, the C/N ratio of the frequency spectrum analyzer is almost equal to the C/N ratio of the first local oscillator 31. Since the first local oscillator 31 is a wide range sweep oscillator, typically a YIG tuned voltage controlled oscillator, which is expensive and is difficult to further decrease its phase noise.